This invention relates to an improved process for the formation of a specific reactive element barrier layer, specifically a protective nitride, carbide, or oxide coating, on a substrate material surface. These coatings are more strongly bonded to that surface than coatings formed by conventional processes.
Nitride, carbide, and oxide coatings have been commercially used as protective coatings which resist corrosion, wear, and erosion. Titanium nitride, because of its excellent tribiological properties, has attracted considerable attention and is probably the most explored and commercialized coating. After titanium nitride, titanium carbide and aluminum oxide have also found wide use.
A useful coating is only as good as the strength of the bond between the coating and the substrate material. Good adhesion is the most important prerequisite toward engineering a commercially useful coating process. For this reason, a number of nitride, carbide, and oxide coating processes have been developed, each attempting to improve the interfacial strength between the coating and the substrate material.
Nitride, carbide, and oxide coating processes in use today include Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), and Ion Assisted Coating (IAV), as well as a combination of these processes.
A problem with conventional coating processes is that the processes can leave contaminants in the interface layer between the coating and the substrate. These contaminants weaken the bond and cause eventual delamination of the coatings.
For example, D'Haen, J. et al. "Interface Study of Physical Vapour Deposition TiN Coatings on Plasma-Nitrided Steels" Surface and Coatings Technology, v 61 (1993), pp. 194-200, reported the formation of iron and chromium nitrides, which are much less stable then titanium nitride, in the interface between an applied titanium nitride coating and the metal during the deposition of a titanium nitride coating. In the present invention, substantially only highly stable, specific reactive element nitrides, carbides, or oxides, for example titanium nitride, are selectively formed on a substrate material surface. Conventional coating processes generally cannot selectively form only a stable specific reactive element nitride, carbide, or oxide coating.
Other contaminants can also drastically reduce the adherence of an applied coating. Muller, D. et al. "Measurement of the Adhesion of TiN and Aluminum Coatings by Fracture Mechanics Tests" Thin Solid Films, v 236 (1993), pp. 253-256, showed that critical load and fracture load, at which point a titanium nitride coating fails, decreases with an increase of oxygen content of the coating. Yet, the presence of oxygen is common in nitride and carbide coatings formed by conventional processes. For example, Baba, K. et al., "Corrosion-Resistant Titanium Nitride Coatings Formed on Stainless Steel by an Ion-Beam Assisted Deposition", Surface and Coatings Technology, v. 66 (1994), pp. 368-372, reported an oxygen content of about 2% in their ion-beam assisted application of titanium nitride films. Rebenne, H. et al., "Review of CVD and TiN Coatings for Wear-Resistant Applications: Deposition Processes and Performances" Surface and Coatings Technology, v.63 (1994) pp. 1-13, reported that after CVD process forming of a titanium nitride coating, the coating contained several atomic percent of chlorine, oxygen and hydrogen. The chlorine and hydrogen are from the TiCl.sub.4 and hydrogen present in the CVD atmosphere. Wu, L. et al., Wear of Materials, "Triboloby, Chemistry, and Structure of Bias Sputtered TiC films on Steel Substrates"; Glaeser et al., Ed., 1977, pp. 364-371, reported that, during the application of a titanium carbide coating, there was a considerable amount of oxygen in their process atmosphere and stated that Auger analysis later revealed the presence of titanium, carbon, oxygen, and iron in the interfacial layer between the titanium carbide and the metal. Wu et al. stated that the presence of oxygen caused the formation of titanium oxide and that delamination always occurred at the titanium oxide/titanium carbide interface.
In the present invention, one process medium used to form nitrides and carbides is pure liquid lithium metal. The liquid lithium does not contain any of the contaminants mentioned above and in addition will reduce and remove all oxides present upon the surface. During the formation of an oxide coating, as well as nitrides and carbides formed in other process mediums, the liquid metal or gaseous environment used is selected to cause only the formation of stable specific reactive element oxides, which substantially excludes the formation of other contaminating compounds.
In conventional coating processes, the processes usually start with a cleaning procedure that uses ion sputtering to remove contaminated compounds, such as sulfides, etc., from the cold substrate material surface that is to be coated. However, during the coating process the substrate material is heated, usually to a temperature between 1100.degree. F. and 2200.degree. F. Any sulfur present in the substrate material will then diffuse from the bulk substrate material to segregate at the surface causing the formation of stable sulfides. These sulfides interfere with the application of the coating and markedly decrease the adherence of the coating.
The solution to this problem, as described in this invention, is to either remove sulfur from the bulk substrate material before forming the coating, or to add a small percentage of a strong sulfide former to the substrate material during its preparation, such as yttrium or hafnium, so that no free sulfur is available to segregate to the substrate surface. One method of initially removing sulfur from the bulk substrate material is to anneal the material in hydrogen at a high temperature, which may be of the order of 2200.degree. F. for superalloys.
The present invention relates to a new and improved technique for forming coatings which overcomes the above-referenced problems and produces a strongly adherent nitride, carbide, or oxide coating.